1. Field of the Invention
This invention relates generally to magnetic random access memory (MRAM) and more particularly to MRAM with multiple layers of magnetic memory cells.
2. Description of the Related Art
MRAM with magnetic tunnel junction (MTJ) memory cells has been proposed for nonvolatile memory, as described in U.S. Pat. No. 5,640,343 and by Reohr et al., “Memories of Tomorrow”, IEEE CIRCUITS & DEVICES MAGAZINE, September 2002, pp. 17–27. In these devices the MTJs are arranged as an array in a single layer (the XY plane) on a semiconductor substrate. In one type of architecture, called a 1T1MTJ MRAM (one transistor and one MTJ), each MTJ is located between a bit line and a transistor, with the word lines located beneath the MTJs. In another type of architecture, called a cross-point (XPC) MRAM, the MTJs are located directly between the bit and word lines. In both MRAM architectures, a selected MTJ cell is “written”, i.e. its magnetic state or +/−X magnetization direction is switched, by write currents passing in X and Y directions through the bit and word lines located above and below the selected MTJ. The write currents generate orthogonal magnetic fields in the X and Y directions that switch the magnetization direction of the selected MTJ. One problem in both MRAM architectures is that the write fields can also switch the magnetization directions in other non-selected MTJs near the selected MTJ. Various solutions to this cell selectivity problem have been proposed, including using MTJs with special geometric shapes, as described in U.S. Pat. No. 6,005,800, and using tunable write currents with different current magnitudes, as described in U.S. Pat. No. 6,683,815.
Stacked or multiple-layer MRAM has also been proposed, but is based on either the 1T1MTJ or XPC architecture for single-layer MRAM, and thus requires bit and word lines above and below the memory cells in each of the memory layers. While solutions have been proposed for the cell selectivity problem in single-layer MRAM, no solutions are known to have been proposed for the cell selectivity problem in multiple-layer MRAM, i.e., writing to a selected cell in one memory layer without also writing to non-selected cells in other memory layers in the stack.
What is needed is an MRAM with multiple memory layers in which a selected cell in one memory layer can be reliably written without also writing to non-selected cells in other memory layers.